U.S. patent number 5,536,964 [Application Number 08/315,835] was granted by the patent office on 1996-07-16 for combined thin film pinhole and semiconductor photodetectors.
Invention is credited to Evan D. H. Green, Tario M. Haniff, Albert K. Hu.
United States Patent |
5,536,964 |
Green , et al. |
July 16, 1996 |
Combined thin film pinhole and semiconductor photodetectors
Abstract
A thin-film semiconductor pinhole component with monolithically
integrated position-sensing photodetectors is herein referred to as
a Position Sensitive Pinhole (PSP). Another embodiment is also
discussed where a PSP is integrated onto a platform with
controllable motion and is herein referred to as a Movable Position
Sensitive Pinhole (MPSP). A third embodiment describes the MPSP
with capacitive, electrostatic actuation incorporated into the
device to achieve controlled motion, herein referred to as a
Capacitively Actuated Movable Position Sensitive Pinhole (CAMPSP).
Each of those embodiments of the present invention are discussed,
as are their method of manufacture and use in a laser environment
as a spatial filter.
Inventors: |
Green; Evan D. H. (San Jose,
CA), Haniff; Tario M. (Berkeley, CA), Hu; Albert K.
(San Jose, CA) |
Family
ID: |
23226272 |
Appl.
No.: |
08/315,835 |
Filed: |
September 30, 1994 |
Current U.S.
Class: |
257/432; 257/433;
250/206.1; 250/237R; 250/208.6; 257/466; 257/443; 257/448;
257/E27.128; 257/E31.115; 257/E27.129 |
Current CPC
Class: |
H01L
27/1446 (20130101); H01L 27/1443 (20130101); H01L
31/02024 (20130101); Y10S 438/977 (20130101); Y10S
438/928 (20130101) |
Current International
Class: |
H01L
27/144 (20060101); H01L 31/02 (20060101); H01L
031/0232 () |
Field of
Search: |
;257/447,448,443,466,432,433,434 ;250/237R,208.6,206.1 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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3910199 |
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Dec 1989 |
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DE |
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4313278 |
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Nov 1992 |
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JP |
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5013808 |
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Jan 1993 |
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JP |
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1848223 |
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Aug 1992 |
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RU |
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Other References
H J. Wolting, Single and Dual-Axis Lateral Photodetectors of
Rectangular Shape, IEEE Transactions on Election Devices, pp.
581-590, (Aug. 1975). .
B. O. Kelly, et al. Techniques for Using the Position Sensitivity
of Silicon Photodetectors to Provide Remote Machine Control, 21st
Annual IEEE Machine Tool Conference, pp. 1-15, (Oct. 1973). .
W. Light, Non-Contact Optical Position Sensing Using Silicon
Photodetectors, United Detector Technology, Inc., Application Note,
pp. 1-24, (Apr. 1982). .
I. Edwards, "Using Photodetectors for Positions Sensing", (Dec.
1988), Sensors. .
Centronic, Inc., Newbury Park, CA, Silicon Photodiodes 1994
Catalog, relevant catalog pp. 1994. .
Newport Corporation, Irvine, CA, Optics Guide 5, pp. 16-18
Including Front and back covers, page bearing Copyright date-1990.
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Newport Corporation, Irvine, CA, Scientific & Laboratory
Products Catalog, pp. 1.14-1.17 including front and back covers and
order info. page, Copyright 1992. .
Y. P. Xu, et al. A Silicon-Diode Based Infrared Thermal Detector
Array, Sensors and Actuators A, 37-38, pp. 226-230, (1993). .
R. F. Wolffenbuttel, Silicon Micromachining for Integrated Radiant
Sensors, Sensors and Actuators A, 36, pp. 109-115, (1992). .
T. M. Haniff, et al., Self-aligning spatial filter, Proceedings of
SPIE, vol. 2220, pp. 6-14, (1994). .
T. Hirano, et al. Design, Fabrication and Operation of Submicron
Gap Comb-Drive Microactuators, Journal of Microelectromechanical
Systems, pp. 52-59, Mar. 1992..
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Primary Examiner: Mintel; William
Attorney, Agent or Firm: Jones; Allston L.
Claims
What is claimed is:
1. A combined thin film pinhole and semiconductor photodetector
array for use with an external electrical biasing source and
current measurement devices in optical applications, said combined
thin film pinhole and semiconductor photodetector array
comprising:
a semiconductor substrate of a selected material defining a pinhole
having a perimeter of a selected size and shape therethrough said
substrate; and
said substrate including thereon an array of a selected number of
photodiodes formed thereon at selected locations around the
perimeter of said pinhole.
2. An optical system comprising:
a combined thin film pinhole and semiconductor photodetector array
having:
a semiconductor substrate of a selected material defining a pinhole
having a perimeter of a selected size and shape therethrough said
substrate; and
said substrate including thereon an array of a selected number of
photodiodes formed thereon at selected locations around the
perimeter of said pinhole; and
an external electrical biasing source connected to said photodiodes
of said semiconductor substrate to back-bias each of said
photodiodes; and
a plurality of current measurement devices connected to said
photodiodes of said semiconductor substrate to detect variations in
individual currents as a result of variations in illumination of
each of said photodiodes.
3. The combined pinhole and semiconductor photodetector array as in
claim 1 wherein said semiconductor photodetector array can
determine spatial relationships between said pinhole and
illumination from a remote light source incident on said
semiconductor photodetector array.
4. The combined pinhole and semiconductor photodetector array of
claim 1 wherein:
said pinhole perimeter shape is selected to be circular; and
said semiconductor substrate is selected to be sufficiently thin to
allow substantial transmission of light through the pinhole.
5. The combined pinhole and semiconductor photodetector array of
claim 4 wherein said pinhole is suitable for optical spatial
filtering.
6. The combined pinhole and semiconductor photodetector array of
claim 1 further includes an electronic means for biasing, current
measurement or voltage measurement of the elements of said
semiconductor photodetector array on said semiconductor
substrate.
7. The combined pinhole and semiconductor photodetector array of
claim 6 wherein said thin film pinhole can be used without external
electrical means for biasing, current measurement or voltage
measurement.
8. The combined pinhole and semiconductor photodetector array of
claim 1 further includes a predetermined number of semiconductor
spring elements which attach the portion of said semiconductor
substrate surrounding said pinhole to the remainder of the
substrate.
9. The combined pinhole and semiconductor photodetector array of
claim 8 wherein said spring elements allow relative motion between
the portion of said semiconductor substrate surrounding said
pinhole and the remainder of the substrate by compression,
extension, or torsion of said spring elements.
10. The combined thin film pinhole and semiconductor photodetector
array of claim 1 further includes:
a predetermined number of semiconductor spring elements which
attach the portion of said semiconductor substrate surrounding said
pinhole to the remainder of the substrate; and
a predetermined number of electrical capacitor structures, each
capacitor structure having:
a predetermined number of electrically accessible elements on the
portion of said semiconductor substrate surrounding said pinhole;
and
a predetermined number of electrically accessible elements on the
remainder of the substrate, separate from said electrically
accessible elements on said portion of said semiconductor substrate
surrounding said pinhole.
11. The combined thin film pinhole and semiconductor photodetector
array of claim 10 wherein said electrical capacitor structures
provides relative motions between the portion of the substrate
containing said pinhole and the remainder of the substrate by
providing force to cause compression, extension, or torsion of said
spring elements.
12. An optical system comprising:
a combined thin film pinhole and semiconductor photodetector array
having:
a semiconductor substrate of a selected material having a pinhole
of predetermined size and shape through said semiconductor
substrate; and
an array of a predetermined number of semiconductor photodetectors
formed on said semiconductor substrate at selected locations around
the perimeter of said pinhole; and
an electronic means for biasing, current measurement or voltage
measurement of the elements of said semiconductor photodetector
capable of detecting variations in the light induced current or
voltage resulting from variations in illumination of each element
of said photodetector array.
13. The optical system of claim 12 wherein said optical system
functions as both an optical element and provides an alignment
signal of said optical element.
14. The optical system of claim 12 further includes an electrically
controlled visual display means for displaying information
describing the variations in illumination of each element of said
semiconductor photodetector array.
15. The optical system of claim 14 wherein said optical system
provides visual feedback to an operator in order to enable said
operator to determine the spatial relationship between said pinhole
and illumination from a remote light source incident on said
semiconductor photodetector array.
16. The optical system of claim 12 further including:
a mechanical mounting means for holding said combined thin film
pinhole and semiconductor photodetector array while allowing
movement of the thin film pinhole relative to a fixed reference
point; and
an electrical computation system means for controlling the motion
of said mechanical mounting means in response to said detected
variations in said current or voltage resulting from said
variations in illumination of each element of said photodetector
array.
17. The optical system of claim 16 wherein said optical system
comprises a means for causing said thin film pinhole and incident
illumination from a remote light source to be brought into
coincidence.
18. A position sensing photodetector array comprising a
semiconductor substrate including multiple photodiode elements,
with each of said multiple photodiode elements:
sharing a common anode or a common cathode with all other elements
of said photodetector array; and
having an anode or a cathode which is separate from all other
elements of said photodetector array; and
having a separate connection to said common anode or common cathode
at a position unique to each of said elements of said semiconductor
photodetector array.
19. The position sensing photodetector array of claim 18 wherein
said position sensing photodetector array provides an electrically
readable means for determining the spatial relationship between
said multiple photodiode elements and incident illumination from a
remote light source.
20. The position sensing photodetector array of claim 18 wherein
said position sensing photodetector array can be switched between
split cell and continuous cell operation conditions by selection of
bias and measurement circuitry appropriate to the selected
operation conditions.
21. The position sensing photodetector array of claim 18 further
defines a pinhole having a perimeter of a predetermined size and
shape through said semiconductor substrate.
22. The position sensing photodetector array of claim 21 wherein
said position sensing photodetector array provides a means for
allowing multiple techniques for determining spatial relationships
between said pinhole and illumination from a remote light source
incident on said semiconductor substrate surrounding said pinhole.
Description
FIELD OF THE INVENTION
The present invention relates to pinholes for use as spatial
filters in laser systems, and more particularly to thin-film
pinholes that optionally include detectors for determining the
position of the pinhole in the light beam and a mechanism to allow
repositioning of the pinhole if it is out of alignment.
BACKGROUND OF THE INVENTION
Pinholes are used as a component in optical systems both as spatial
filters to remove unwanted variations in light intensity across a
light beam (typically if the light is generated from a laser) and
as alignment aids to insure that incident light impinges on other
optical elements at locations determined by the position of the
pinhole. Thus, pinholes often must be placed in optical systems at
precisely specified locations. A limitation in the usefulness of
pinholes is that the exact position of prior art pinholes is
difficult to establish initially and subsequently maintain from
drifting, making systems which incorporate pinholes as either
spatial filters or alignment aids difficult to align and prone to
long-term performance degradation as the position of the pinhole
drifts.
In a typical prior art pinhole component, the pinhole is
laser-drilled through a metal foil. The prior art pinhole is
therefore a passive device, incorporating no elements capable of
measuring incident light intensity or position and further being
incapable of straightforward modification to add position sensing
elements. Typical metal foil thickness of 10-12 .mu.m is used for
these pinhole devices. However, pinholes smaller in diameter than 5
.mu.m, are typically drilled through 6 .mu.m thick foils. The use
of thin metal foils results in fragile components which are prone
to damage and warping during normal handling and use.
One embodiment of the present invention includes a thin-film
position sensitive photodetector element that is integrated with a
pinhole. There are two types of typical prior art position
sensitive photodetector elements, each with possible variations on
the technique for formation of the photodetector. One is a
split-cell type position sensitive photodetector element that
consists of multiple planar semiconductor junctions, adjacent but
separated by gaps as small as 1 .mu.m. In operation, when light is
incident on the split-cell photodetector, different currents will
be sensed in each of the semiconductor junctions of the split-cell.
A second is a continuous-cell type of position sensitive
photodetector element that consists of a single planar
semiconductor junction with multiple contacts at its perimeter. In
operation, when light is incident on the continuous-cell position
sensitive photodetector, different currents will be sensed in each
of the perimeter contacts depending on the relative distance
between the centroid of the incident light and the perimeter
contacts.
Each of the types of position sensitive photodetectors has a
different practical application. The split-cell type is most useful
for measuring small deviations in the location of an optical beam
about a location centered on the gap between elements. The
continuous-cell type is most useful for providing an approximately
linear change in output signal with the changes in the position of
a light beam. This is accomplished by noting the current difference
from side to side of the entire photodetector element semiconductor
junction. As compared to the spit-cell type, the continuous-cell
type measures a larger range of incident light locations with less
precision than the split-cell photodetector.
For either type of position sensitive photodetector, the
semiconductor junctions are formed as metal-semiconductor junctions
(Schottky-type photodetector), p-n semiconductor junctions, or
p-i-n semiconductor junctions with the overall device operating as
discussed above for any of the junction structures. In the
formation of a metal-semiconductor junction, the semiconductor
surface is coated with metal (typically less than 10 nm thick). In
the p-n semiconductor junction, the manufacturing process
introduces a layer, typically less than 1 .mu.m thick, of p-type
dopant into an n-type semiconductor substrate (or alternately
n-type into p-type). In the p-i-n semiconductor junction, the
typical manufacturing process is to epitaxially grow a layer
(typically more than 10 .mu.m thick) of lightly doped (near
intrinsic) semiconductor on top of an n-type substrate, then
introduce a layer of p-type dopant (typically less than 1 .mu.m
thick) into the epitaxial layer. In all of the cases described
above, the substrate is derived from a semiconductor wafer, and the
total device thickness is determined by the thickness of the
semiconductor wafer with the wafer thickness being quite variable,
i.e. typically 300 .mu.m or more.
In summary, the prior art includes, independently, pinhole plates
and position sensitive photodetectors. As discussed above, pinholes
are manufactured in thin metal foils with the thin foil making the
drilling process possible, and allowing for efficient pinhole
operation, however, the pinhole foils are subject to mechanical
damage and do not incorporate position sensitive elements. Also, as
discussed above, the prior art includes position sensitive
photodetectors that are fabricated in relatively thick substrates,
hence they are not amenable to combination with the manufacture of
a pinhole foil. What is needed is a pinhole and method of
manufacture that overcomes the limitations of the presently
available art and permits the integration of position sensitive
photodetectors with pinhole assemblies. The present invention
provides such a device and method of manufacture.
SUMMARY OF THE INVENTION
In accordance with the present invention there is shown a combined
thin film pinhole and semiconductor photodetector array for use
with an external electrical biasing source and current measurement
devices in optical applications. The combined thin film pinhole and
semiconductor photodetector array of the present invention includes
a semiconductor substrate of a selected material that defines a
pinhole therethrough of a selected size and shape with an array of
a selected number of photodiodes formed on the substrate at
selected locations around the perimeter of the pinhole.
The present invention also discloses a method of manufacture of a
combined thin film pinhole and semiconductor photodetector array
substrate for use with an external electrical biasing source and
current measurement devices in optical applications. That method of
manufacture includes a variety of steps including initially
fabricating a semiconductor substrate having a first and a second
semiconductor layer one on top of the other wherein the top layer
is of a selected thickness as required for the optical application
and each of the layers have a top surface and a bottom surface. A
top protective layer is developed on the top surface of the top
layer and a bottom protective layer on the bottom surface of bottom
layer of the fabricated substrate. Also, selected windows are
opened through the top protective layer through which a selected
impurity is inserted, followed by the redevelopment of the top
protective layer. Additionally, a window is opened through the
bottom protective layer through which the bottom layer of the
substrate is etched to form a cavity with the lateral extent of the
cavity being determined by the crystal plane orientation of the
bottom layer which intersect the third window and the bottom
surface of said top layer. Further, contact windows are opened
through the protective layer within each of the windows in the top
layer to permit the deposition a selected conductive material on
the substrate and into the contact windows to make external
electrical connection to those buried layers. And to open another
window through the top protective layer at a selected location
above the location of the cavity to permit the etching of the
pinhole through the substrate.
There is also disclosed an optical system that has a combined thin
film pinhole and semiconductor photodetector array having on a
semiconductor substrate with a pinhole defined therethrough, and an
array of a selected number of photodiodes formed thereon at
selected locations around the perimeter of the pinhole. The system
further includes an electrical biasing source external to the
substrate that is connected to the photodiodes on the substrate to
back-bias each of the photodiodes, and a plurality of current
measurement devices connected to the photodiodes to detect
variations in individual currents as a result of variations in
illumination of each of the photodiodes.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view of a typical prior art laser-drilled
metal pinhole component.
FIGS. 2a and 2b show cross-sectional projections of the two typical
prior art position sensitive photodetectors.
FIG. 3 shows a cross-sectional projection of a bonded compound
semiconductor substrate used in the fabrication of the present
invention.
FIGS. 4a and 4b show different cross-sectional projections of a
quadrant of the semiconductor substrate of FIG. 3 after
introduction of an n+ conductivity layer.
FIGS. 5a and 5b show the cross-sectional projections of a portion
of the semiconductor substrate of FIGS. 4a and 4b, respectively,
after introduction of the p+ conductivity layer to form a
photodetector junction.
FIGS. 6a and 6b show cross-sectional projections of a portion of
the semiconductor substrate of FIGS. 5a and 5b, respectively, after
etching of a back-side cavity.
FIGS. 7a and 7b show cross-sectional projections of a portion of
the semiconductor substrate of FIGS. 6a and 6b, respectively, after
etching of contact windows.
FIGS. 8a and 8b show cross-sectional projections of a portion of
the semiconductor substrate of FIGS. 7a and 7b, respectively, after
deposition and patterning of a metal layer.
FIG. 9 shows cross-sectional projections of a portion of the
semiconductor substrate of FIG. 8a after pinhole etching and
optional back-side metallization.
FIG. 10a shows a top plan view of a completed PSP component of the
preferred embodiment, made up of 4 symmetric quadrants.
FIG. 10b is a cross-sectional view of the substrate shown in FIG.
10a taken along broken line 10b--10b.
FIG. 10c is an electrical schematic representation of the device of
FIG. 10a.
FIG. 10d is an electrical schematic representation of the device of
FIG. 10c under split-cell operating mode.
FIG. 10e is an electrical schematic representation of the device of
FIG. 10c under continuous-cell operating mode.
FIGS. 11a-11b are cross-sectional projections of the semiconductor
substrate of FIGS. 4a and 4b as modified to form a movable position
sensitive pinhole (MPSP).
FIG. 11c shows a top plan view of a completed MPSP component of the
preferred embodiment, made up of 4 symmetric quadrants.
FIGS. 12a-12b are cross-sectional projections the semiconductor
substrate of FIGS. 4a and 4b as modified to form a capacitively
actuated movable position sensitive pinhole (CAMPSP).
FIG. 12c shows a top plan view of a completed CAMPSP component of
the preferred embodiment, made up of 4 symmetric quadrants.
DESCRIPTION OF THE PREFERRED EMBODIMENT
FIG. 1 is a perspective view of a typical prior art laser-drilled
pinhole component that is a substantially circular thin metal foil
substrate 100. Pinhole 102 is typically created by exposing
substrate 100 to intense laser radiation. As a result of the high
temperatures induced in substrate 100 while drilling hole 102
therethrough, substrate 100 unavoidably melts in that vicinity,
resulting in ragged edges 104 that surround pinhole 102.
FIGS. 2a and 2b show cross-sectional projections of typical prior
art position sensitive photodetectors. The device of FIG. 2a is a
split-cell photodetector with separate p+ doping type anode areas
110 and 112 that each respond to incident optical beams on the left
or right of a dividing line formed between anodes 110 and 112. In
this configuration, anodes 110 and 112 have a shared cathode 114 in
an n-type semiconductor substrate 116. The device of FIG. 2b is a
typical prior art continuous-type position sensitive photodetector
with a single anode 120 and left and right cathode sections 122 and
124 in a common substrate 126.
The following description discusses the structure of a pinhole
component of the present invention with monolithically integrated
position-sensing photodetector element as a semiconductor device
hereinafter referred to as Position Sensitive Pinhole (PSP). The
steps to be described of fabricating the position-sensing
photodetector surrounding a pinhole etched through the
semiconductor wafer can be performed in different orders of various
steps. It is also possible to reorder the many sub-steps of many of
the steps to achieve the same result. Many of the described
manufacturing steps additionally may be replaced by essentially
equivalent alternatives without significantly changing the device
structure. However, to maximize the component yield from the
substrate, and minimize the manufacturing costs of each substrate,
it is desirable to maintain planar top and bottom surfaces of the
substrate that are pinhole free through as much of the
manufacturing process as possible. Therefore, the preferred order,
given today'technology, is to complete the fabrication steps for
the position-sensing photodetector element of the finished device
before a back cavity is etched, and subsequently, the pinhole
etched through the thinned portion of the substrate.
The following discussion illustrates a particular implementation of
a pinhole integrated with a quadrant split-cell photodetector by
the preferred technique. At several steps in the manufacturing
sequence, alternative manufacturing processes are mentioned,
however, many of the other steps may also be performed by
alternative processes. The type of position sensitive photodetector
may be selected from either of the prior art types, i.e. split-cell
types (of which a quadrant cell is one example) and continuous-cell
types with simple modifications to the described technique, thus
making it possible to integrate any number or type of desired
position sensing elements with the thin film pinhole of the present
invention. That flexibility makes it possible to simultaneously
implement a position sensitive photodetector variation that
incorporates both of the continuous and split-cell types. It should
also be noted that the thicknesses of the various layers in the
accompanying figures are not to scale and have been drawn as they
have to illustrate the steps in the manufacturing process with one
skilled in the art being in a position to realize the actual
thickness of each such layer.
FIG. 3 illustrates the starting point in the fabrication of each of
the embodiments of the present invention. As shown in FIG. 3 there
is a compound semiconductor substrate consisting of top and bottom
semiconductor wafers 26 and 30 that are bonded to each other via an
intermediate bonding layer 28 (e.g. a thermally grown silicon
dioxide layer). The preferred bonding technique is a widely known
thermal silicon bonding technique. Alternatively, the substrate of
FIG. 3 can be created by an oxygen-implantation and epitaxial
growth technique known as Simox. To facilitate the etch
orientations of later steps, both of wafers 26 and 30 will
typically be selected of (100) crystalline orientation with wafer
26 additionally typically being a high resistivity n- conductivity
type silicon. Wafer 30 can be of standard available thickness,
typically 525 .mu.m for four-inch diameter silicon wafers. Wafer 26
should be thinned for optimal optical performance of the pinhole,
and may be thinned after thermal bonding to between 1 and 10 .mu.m.
The compound substrate of FIG. 3 has four semiconductor surfaces,
the top-wafer-top-surface 36, the top-wafer-bottom-surface 38, the
bottom-wafer-top-surface 34, and the bottom-wafer-bottom surface
32.
From this point through the balance of the discussion of the
fabrication steps of a PSP of the present invention it will be
assumed that the finished device will have four identical sections,
or quadrants. However, the number of sections that the finished
device of the present invention may have can be any number, with
the minimum number of sections being one, The number of sections
selected is dictated by the overall accuracy desired in the
determination of the position of a light beam relative to the
pinhole of the finished device. In the other embodiments of the
present invention the number of sections of the finished device
will also be determined by how much flexibility is desired in the
ability to reposition the pinhole relative to the light beam.
FIGS. 4a and 4b each show a different cross-sectional projection of
the compound semiconductor substrate from FIG. 3 after growth or
deposition of a top protective layer 14 (typically SiO2) on
top-wafer-top-surface 36 and a bottom protective layer 44 on
bottom-wafer-bottom-surface 32. A window 13 is then etched into top
protective layer 14 using known photomasking and etching
techniques. Window 13 and top protective layer 14 determine the
location of n+ conductivity layer 40, formed by introduction of n-
type impurity (typically phosphorous) into top-wafer-top-surface 36
of top wafer 26 by known processes (e.g. dopant diffusion or ion
implantation).
FIGS. 5a and 5b show the same cross-sectional projections of the
compound semiconductor substrate as in FIGS. 4a and 4b,
respectively, after a new top protective layer 12 is re-grown or
re-deposited over the shallow diffused n+ conductivity layer 40
within window 13, and windows 11 are etched into the top protective
layer 14 using known photomasking and etching techniques. Window 11
and top protective layers 14 and 12 determine the location of p+
conductivity layer 42, formed by introduction of p-type impurity
(typically boron) into top-wafer-top-surface 36 of top wafer 26 by
known processes (e.g. dopant diffusion or ion implantation). This
p+ conductivity layer 42 defines a position sensitive photodetector
element.
FIGS. 6a and 6b show the cross-sectional projections of the
compound semiconductor substrate of FIGS. 5a and 5b after a top
protective layer is re-grown or re-deposited in windows 11,
creating a new protective layer 10 over the shallow ion implanted
p+ conductivity layer 42. Subsequently, a window 43 is etched
through bottom protective layer 44 to expose
bottom-wafer-bottom-surface 32 of wafer 30 using known chemical
etch techniques. Wafer 30 is then etched using known chemical etch
techniques with bottom protective layer 44 acting as an etch mask
and the cavity boundary 45 typically defined by the (111) crystal
planes of bottom semiconductor wafer 30 which intersect the etch
mask window 43. The back cavity 46 etch consumes the entire
thickness of the bottom wafer 30, and is easily prevented from
accidental etching top wafer 26 by choosing a chemical solution
which preferentially etches semiconductor wafer 30 and not
interface layer 28. For silicon semiconductor wafers and SiO2
interface layers, many chemical solutions with the above described
etch properties exist, most widely used are potassium hydroxide
(KOH), ethylene diamine pyrocatechol (EDP), and tetramethyl
ammonium hydroxide (TMAH). Alternative manufacturing techniques
where a bonding layer 28 is not used in the compound substrate can
use electrochemical etch stop, dopant selective etching, or
controlled, timed etching from the bottom surface of any common
semiconductor wafer.
FIGS. 7a and 7b show the cross-sectional projections of the
compound semiconductor substrate from FIGS. 6a and 6b when
protective layer 12 in window 13 is photomasked and etched to
create a contact window 22 to the top surface of n+ conductivity
layer 40, and protective layer 10 in windows 11 is photomasked and
etched to create a contact window 18 to the top surface of p+
conductivity layer 42. Contact windows 18 and 22 are defined on a
single photomask referred to as the contact window photomask.
FIGS. 8a and 8b show the cross-sectional projections of the
compound semiconductor substrate from FIGS. 7a and 7b after a metal
(typically aluminum) is deposited by known evaporation or
sputtering techniques over the protective layers 10, 12, and 14,
and into the contact windows 18 and 22. The metal is then
photomasked and etched to leave substrate contact traces 24 and
photodetector contact traces 20. The photomask defining the contact
traces 20 and 24 is referred to as the metal etch photomask.
FIG. 9 shows the cross-sectional projections of the compound
semiconductor substrate from FIG. 8a after a pinhole 16 is etched
through the protective layer 14, the top wafer 26, and the
intermediate bonding layer 28 to cavity 46 by known masking and
chemical etching techniques. The extent and location of pinhole 16
are controlled by photomasking the entire top surface of the
substrate and through that photomask, and through protective layer
14, creating window 17. The shape of the side-walls of pinhole 16
through each layer is determined by the etch technique selected. A
typical choice of isotropic etch, with directionality normal to the
etched surface 36 will result in a pinhole shape closely described
by the shape of window 17. Example etch techniques for pinhole 16
are C12 or SF6 containing plasma etch recipes, and are widely used
in DRAM capacitor etching. Alternative choices of crystal
orientation selective etchants, such as those described in
conjunction with the back cavity 46 etch may be used to create
square or rectangular pinholes, determined by the intersections of
the (111) crystal planes of top wafer 26 and the etch window 17.
Another alternative pinhole etch is anisotropic semiconductor
etchants (e.g. Hydrofluoric-Nitric-Acetic acid mix) which results
in pinhole 16 having sloped sides. After pinhole etch, a metal
layer 48 (typically aluminum) may optionally be evaporated on the
overall bottom surface of the completed substrate, although metal
layer 48 is not required for the electrical operation of the
device. Metal layer 48 will, however, enhance photodetector
response to long wavelength light, and reduce unwanted transmission
of light through the thin top wafer 26.
FIG. 10a shows a top plan view of the complete semiconductor
substrate including four quadrants of the configuration shown in
FIG. 9 after PSP fabrication has been completed. The broken line
segments between arrows marked 9--9 indicate the quartering
cross-section used for the cross-section projections of a single
quadrant in FIGS. 4a, 5a, 6a, 7a, 8a, 9, 11, and 12. Similarly, the
broken line segment between arrows marked 8b--8b indicate the
cross-section axis used for the cross-section projections of FIGS.
4b, 5b, 6b, 7b and 8b. From FIG. 10a it can be seen that the
regions that divide each of the quadrants are portions of the
connected top protective layer 14.
After the completed PSP component is separated from the remainder
of the substrate, that portion of the bottom wafer which was not
removed in the etching of the cavity now forms a solid frame
surrounding the membrane of the top wafer above the etched
cavity.
FIG. 10b shows a cross-sectional view of the device of FIG. 10a
with the cross-section dividing the device in half through pinhole
16 and top protective layer 14 as viewed from the top of the
substrate as in FIG. 10a. This view is provided to show the
physical relationship between adjacent quadrants of the finished
device.
The position sensitive photodetector embodiment shown in FIG. 10a
has four anodes (a split-cell configuration with four separate
anodes forming four separate photodiodes) defined by the four p+
conductivity areas 42 (under protective layer 10). Additionally,
the current embodiment has multiple cathode connections defined by
the four n+ conductivity areas 40 (under protective layer 12),
connected to a single cathode defined by the top semiconductor
wafer 26, as typical in a continuous-cell configuration. This
embodiment of the position sensitive photodetector is a hybrid of
split-cell and continuous-cell types, because the defining feature
of a split-cell configuration is to have multiple anodes, while the
defining feature of a continuous-cell configuration is to have
multiple connections to a single cathode.
For the present embodiment, the operating principle is selectable
by user applied biasing conditions. With simple modifications to
the photomasks defining the p+ conductivity areas or the n+
conductivity areas, the position sensitive photodetector element
may be manufactured to be of typical split-cell or continuous-cell
type.
FIG. 10c schematically illustrates the electronic characteristics
of the quad PSP device of FIG. 10a with the physical structures
pictured in previous figures replaced here by their electrical
circuit symbols. The dot-dashed box 202 is the physical outline of
a PSP chip of the present embodiment. The individual photodiode
cells 210 are shown surrounding the etched pinhole 16 with separate
photodetector contact traces 20 and separate substrate contact
traces 24 to each of the four individual photodiode cells 210
around the circumference of the chip. The physical outline of top
semiconductor wafer 26 is shown in this schematic representation as
dotted polygon 204, since top semiconductor wafer 26 forms a common
cathode between all four photodiode cells 210, provides a
resistance network that is connected to each of the cathodes of
photodiodes 210. That resistive network is shown having two
sections: a bridge of equivalent resistors 206 surrounding pinhole
16 with a each node of that bridge connected to the cathode of a
different one of the four photodiodes 210; and individual resistive
paths 208 between the individual cathodes of the four photodiodes
210 of conductive wafer 26 and the n+ conductivity layer 40 to
which substrate contact traces 24 are connecting and outward around
the periphery of the substrate.
As discussed with respect to the prior art, there are typical two
operating modes of position sensitive photodetector elements,
split-cell operation and continuous-cell operation, either of which
can be obtained with the PSP of the embodiment of FIG. 10c.
Additionally, if a mixed operation of both split-cell and
continuous-cell were desired, that too is possible with the PSP
embodiment of FIG. 10c since the necessary interconnections between
the anodes or the cathodes of photodiodes 210 for the operational
modes are not included on the substrate.
FIGS. 10d and 10e are provided to illustrate the split-cell and
continuous-cell operational modes, respectively, for the quad-PSP
of FIG. 10c.
In FIG. 10d a simplified schematic of the PSP substrate 202 of FIG.
10c is shown as a block with pinhole 16 through the center and the
four pairs photodetector and substrate contact traces 20 and 24,
respectively, around the perimeter of substrate 202. To implement
the split-cell operational mode, as shown here, each of the four
substrate contact traces 24 are shorted together and connected to
the positive terminal of an applied voltage source 200. Each of
photodetector contact traces 20, on the other hand, are serially
connected through individual current measurement devices 212 to the
negative terminal of applied voltage source 200 thus reverse
biasing each of photodiodes 210.
In this configuration, the measured current in each of the current
measurement devices 212 is therefore equal to the current through
the corresponding photodiode cell 210 to which it is connected. In
this, the split-cell operating mode, each photodiode cell 210
generates a current proportional to the incident light power
received by that cell, and from the individual currents measured by
current measurement devices 212 each current can be compared to
each other current to determine the position of the light beam
relative to each of the photodiode cells. For example, if the light
beam is at the top of pinhole 16, the photodiode that is located
adjacent that point will conduct the largest current, the
photodiodes on either side of pinhole 16 will conduct the same
current level which is less than the current level flowing in the
top photodiode, and the photodiode adjacent the bottom of pinhole
16 will conduct the least current since it furthest from the light
beam. Similarly if the light beam is intermediate two photodiodes,
those two photodiodes will conduct the highest currents with the
current conducted by each being proportional to the distance to the
light beam, and the other two photodiodes will conduct less current
in proportion to the distance from the light beam. Thus, through a
calibration of the various currents in each of the diodes the exact
position of the light beam with respect to the center of pinhole 16
can be determined. It should be noted at this point in the
discussion that the number of photodiodes that can be included in a
device of the present invention can be any number, and that the
accuracy by which the position of the light beam can be determined
is directly proportional to the number of photodiodes surrounding
pinhole 16.
FIG. 10e illustrates the biassing of the PSP embodiment of the
present invention in the continuous-cell mode. Here, as in FIG.
10d, a simplified schematic of the PSP substrate 202 of FIG. 10c is
shown as a block with pinhole 16 through the center and the four
pairs photodetector and substrate contact traces 20 and 24,
respectively, around the perimeter of substrate 202. To implement
the continuous-cell operational mode each of the four photodetector
contact traces 20 are shorted together and connected to the
negative terminal of an applied voltage source 200. Each of
substrate contact traces 24, on the other hand, are serially
connected through individual current measurement devices 212 to the
positive terminal of applied voltage source 200. Connected in this
way, each of the four photodiodes 210 are reverse biased by bias
voltage 200, as they were in the split-cell operating mode
illustrated in FIG. 10d.
Connected in this way, as can be seen by referring to both FIG. 10e
and FIG. 10c, the top semiconductor wafer 26 forms an electrically
resistive paths 206 and 208 between current measurement devices 212
and the photodiode cell cathodes. Thus, generated current divides
between the cathode connections in inverse proportion to the
resistance of each path, i.e. the current flow is greater the lower
the resistance of the path.
Continuing that thought, the greater the intensity of the light
incident on a photodetector 210, the more current that flows
therethrough, thus, in resistive terms, causing the photodiode to
appear to have a lowered resistance. When viewing only one quadrant
of the schematic circuit of FIG. 10c it can be seen that photodiode
210, resistor 208 and current measurement device 212 are serially
connected across applied voltage source 200. Accordingly, the
voltage at the cathode of photodiode 210 increases causing current
to flow to the other photodiodes 210 and reduces the current that
flows through each of them. That influence by one photodiode on
each of the others is also influenced by the intensity of the light
incident on the other photodiodes 210 as a result of the resistive
bridge of resistors 206. Therefore, observation of the current
difference between each of the anode connections leads to a
conclusion about the relative location of the centroid of the
incident optical power and the anode connections. In this
continuous-cell scheme, the current difference observed as above
results not only in information about the direction of offset
between the center of pinhole 16 and the incident light location,
but also in information about the offset distance from each of
photodiodes 210.
Using either the split-cell or the continuous-cell operating mode,
the photodiode cell 210 Generating the greatest current can be
concluded to be that one which is receiving the greatest light
power. Such a conclusion made, a typical operating decision would
be to mechanically reposition the PSP by moving it in the direction
of that photodiode cell which is receiving the greatest light
power. If the measurement and repositioning sequence is continually
repeated, eventually the PSP will be positioned so that the
incident optical power passes through the center of pinhole 16. In
operating applications either as a beam-aligned or a spatial
filter, the use of biasing and measurement circuitry and a
mechanical repositioning technique allows the PSP to be aligned
concentric with a light beam.
The hybrid nature of the PSP position sensitive photodetector cells
of the present invention described above results in a position
sensitive photodetector which can be switched at will from the
high-certainty offset direction finding operation of a split-cell
to the high-accuracy offset distance measurement operation of a
continuous-cell. With flexible biasing circuitry, the results of
both operating modes can be obtained with reconfiguration switching
circuits that permit the switching back and forth between the two
modes in rapid succession. Such an operating mode results in the
generation of more information of the relative locations of the
photodetector and the incident light than possible with either the
fixed split-cell or the fixed continuous-cell types of position
sensitive photodetectors alone.
The second embodiment of the present invention is a Movable PSP
(MPSP) to facilitate the positioning of the pinhole either
initially, or during operation when a positional shift of the
pinhole has occurred. The process steps to produce a device of this
type is similar to that of the PSP. To illustrate the process for
production of an MPSP is discussed by using the cross-sectional
projection of a single quadrant of device at different stages of
the production. As with the PSP, it should be remembered that the
use of a quadrant structure is for illustrative purposes only, and
that the finished device may have as many sectors as desired.
Referring first to FIG. 11a there is shown a cross-sectional
projection of a single quadrant of a partially completed Movable
PSP (MPSP) embodiment. To reach this point, the sequence of
manufacturing steps leading to FIG. 11a is the same as previously
described for manufacture of a PSP. Comparing FIG. 11a with FIG.
4a, window 13' compares to window 13 with an n+ conductivity layer
40' beneath new top protective layer 12'. Similarly, when FIG. 11a
is compared to FIG. 5a, window 11' compares to window 11 with a p+
conductivity layer 42' beneath new protective layer 10'.
In window 11', two of the window defining features 60 and 62 define
a region 64 between them where spring structure 50 (see FIG. 11b)
is to be formed.
Next, by comparing FIGS. 11a and 6a, it can be seen that here that
etch mask window 43' has been sized and shaped so that the etched
cavity boundary 45' in the bottom of the device is substantially
aligned with window defining feature 60 on the top of the device.
The creation of cavity 46' is created in the same way described for
the creation of cavity 46 for the PSP embodiment. Then in the same
way as described in relation to FIGS. 7a and 8a, contact windows
18' and 22' are opened in the regions within windows 10' and 12',
followed by the metalization of photodetector contact trace 20' and
substrate contact trace 24'.
To obtain the configuration of FIG. 11b from that of FIG. 11a
requires several steps that are similar to the steps illustrated in
FIG. 9 for the PSP embodiment. First, a window in new protective
layer 10' in region 64 and window 17 in top protective layer 14 are
opened. Then the substrate is etched completely through in those
regions to form spring 50 and pinhole 16. Spring 50 interconnects
the thicker perimeter portion 66 of the substrate with the thinner
floating platform 52 in the interior of the substrate, platform 52
substantially incorporates the PSP structure of the previous
embodiment. Note here that spring 50 is shown here having a single
loop, however, additional loops could be created if desired to
permit greater movement of platform 52 to orient pinhole 16
relative to the light beam directed through it. Additionally, note
that photodetector contact trace 20' is electrically interconnected
with p+ conductivity layer 42' beneath new protective layer 10'
since p+ conductivity layer 42' extends through spring 50 in top
wafer 26.
FIG. 11c shows a top plan view of a completed, four-quadrant MPSP
of the second embodiment of the present invention. To assist in the
visualization of this structure first focus on the fact that areas
53 are voids, or removed semiconductor material areas that extend
completely through the substrate. Thus, there are three general
areas of the MPSP substrate: the thinned platform 52 that defines
pinhole 16 centrally therethrough; the thick continuous perimeter
portion 66 that encircles platform 52; and four springs 50
connected to the inner edge of perimeter portion 66 that
interconnect, mechanically and electrically, and provide the sole
support for platform 52.
This configuration allows platform 52 to be moved in any planar
direction separately from the perimeter portion 66 of the
substrate. The standard operation for a MPSP is therefore to
measure the position of incident light relative to the pinhole 16
using the PSP function of platform 52, using either split-cell or
continuous-cell techniques described previously. Then, using that
measured position information, circuitry external to the MPSP can
be used to cause an externally supplied actuation mechanism to move
the MPSP platform in such direction as to reduce pinhole to light
beam position offset.
The third embodiment of the present invention is a capacitively
actuated MPSP (CAMPSP) to automatically facilitate the positioning
of the pinhole either initially, or during operation when a
positional shift of the pinhole has occurred. The process steps to
produce a device of this type is similar to that of the MPSP. To
illustrate the process for production of an CAMPSP is discussed by
using the cross-sectional projection of a single quadrant of device
at different stages of the production. As with the PSP, it should
be remembered that the use of a quadrant structure is for
illustrative purposes only, and that the finished device may have
as many sectors as desired.
Referring first to FIG. 12a shows a cross-sectional projection of a
single quadrant of a partially completed capacitively actuated MPSP
(CAMPSP) embodiment. The fabrication steps that are necessary to
achieve the configuration of FIG. 12a are substantially the same as
those necessary to achieve the configuration of the MPSP device in
FIG. 11a. Comparing FIG. 12a with FIGS. 4a and 11a, window 13"
compares to windows 13 and 13' with an n+ conductivity layer 40"
beneath new top protective layer 12". Similarly, when FIG. 11a is
compared to FIGS. 5a and 11a, window 11" compares to windows 11 and
11' with a p+ conductivity layer 42" beneath new protective layer
10". Additionally, FIG. 12a includes a third window 57 through top
protective layer 14 that includes a thinned protective layer 58
covering a p+ conductivity layer 56 (see FIG. 12b) which is
identical to, but not connected to, p+ conductivity layer 42
beneath new protective layer 10". Since p+ conductivity layers 56
and 42 are identical, p+ conductivity layer 56 can be introduced
during the same manufacturing step that p+ conductivity layer 42 is
introduced. Similarly, protective layers 10" and 58 can be grown
during the same step as well.
Additionally, a window is opened in protective layer 58 when
windows 18" and 22" are opened, with an actuator control metal
trace 54 being introduced contact window 55 to make connection with
p+ conductivity region 58. This operation is similarly performed
during the metalization step in which contact traces 20" and 24"
are created.
In window 11", two of the window defining features 60 and 62'
define a region 64 between them where spring structure 50 (see FIG.
12b) is to be formed. Window defining feature 62' in FIG. 12a is a
"U" shaped structure that extends across new protective layer 10"
which also defines within that "U" shape an finger like extension
of p+ conductivity layer 56 beneath protective layer 58 to form a
capacitive finger 70 (see FIG. 12b) between spring 50 and platform
52' after the fabrication of the CAMPSP device is completed.
Next, by comparing FIGS. 12a, 11a and 6a, it can be seen that here
that etch mask window 43' has been sized and shaped so that the
etched cavity boundary 45' in the bottom of the device is
substantially aligned with window defining feature 60 on the top of
the device. Cavity 46" is created in the same way described for the
creation of cavity 46 for the PSP embodiment. Then in the same way
as described in relation to FIGS. 7a and 8a, contact windows 18",
22" and 55 are opened in the regions within windows 10", 12" and
57, followed by the metalization of photodetector contact trace
20", substrate contact trace 24" and control metal trace 54.
To obtain the configuration of FIG. 12b from that of FIG. 12a
requires several Steps that are similar to the steps illustrated in
FIG. 9 for the PSP embodiment and those discussed with respect to
FIG. 11b for the MPSP embodiment. First, a window in new protective
layer 10" in region 64 that extends around "U" shaped feature 62',
and window 17 in top protective layer 14 are opened. Then the
substrate is etched completely through in those regions to form
spring 50', capacitive finger 70 and pinhole 16. Spring 50'
interconnects the thicker perimeter portion 66' of the substrate
with the thinner floating platform 52' in the interior of the
substrate, with capacitive finger 70 intermediate platform 52' and
the adjacent side of spring 50'. Here, as in the MPSP embodiment,
platform 52' substantially incorporates the PSP structure of the
first embodiment. Note that spring 50' is shown here having a
single loop, however, additional loops could be created if desired
to permit greater movement of platform 52' to orient pinhole 16
relative to the light beam directed through it. Additionally, note
that photodetector contact trace 20" is electrically interconnected
with p+ conductivity layer 42" beneath new protective layer 10"
since p+ conductivity layer 42" extends through spring 50' in top
wafer 26. Additionally, FIG. 12b shows p+ conductivity layer 56
extending throughout capacitive finger 70.
FIG. 12c shows a top plan view of a completed four-quadrant CAMPSP
of the third embodiment of the present invention. To assist in the
visualization of this structure first focus on the fact that areas
53' and 68 are voids, or removed semiconductor material areas that
extend completely through the substrate. Thus, there are four
general areas of the CAMPSP substrate: the thinned platform 52'
that defines pinhole 16 centrally therethrough; the thick
continuous perimeter portion 88' that encircles platform 52'; four
springs 50' connected to the inner edge of perimeter portion 66'
that interconnect, mechanically and electrically, and provide the
sole support for platform 52'; and four capacitive fingers 70, one
in each quadrant, intermediate platform 52' and spring 50'.
In order to function as one plate of a capacitive actuation
mechanism, p+ conductivity layer 56 must be confined to an area
outside of platform 52', separated by etched semiconductor area
53'. Using the separate actuator control metal trace 54 and
substrate contact trace 24", a voltage difference can be applied
between p+ conductivity layer 56 outside of platform 52' and top
semiconductor wafer 26 of platform 52'. Such a voltage difference
necessarily results in capacitive attraction between p+
conductivity layer 56 and platform 52' by electrostatic forces. The
CAMPSP embodiment therefore is manufactured in such a way that it
contains both an element (a PSP) which can detect the relative
position of pinhole 16 and an incident light beam and an element
which can move the platform 52' which contains pinhole 16. The
standard operation for a CAMPSP will therefore be to measure
position of incident light relative to pinhole 16, using either
split-cell or continuous-cell techniques described previously. From
the measured position information, circuitry external to the MPSP
can be used to apply a voltage difference between the desired
control metal trace 54 and substrate contact trace 24", causing
platform 56' to move towards electrically charged p+ conductivity
layer 58' in such direction as to reduce pinhole to light beam
position offset.
After a completed PSP, MPSP, or CAMPSP component is separated from
the remainder of the substrate, that portion of the bottom wafer
which was not removed in the etching of the cavity now forms a
solid frame surrounding the pinhole. In the MPSP and CAMPSP
embodiments, the solid frame surrounds a platform containing the
pinhole, which platform is tethered to the remaining substrate of
tile separated MPSP or CAMPSP component. In the CAMPSP embodiment,
the platform is adjacent to capacitor plates which can be used to
cause platform movement in response to the detected position of a
light beam relative to the pinhole.
Although the description above primarily discusses the use of
presently known fabrication techniques, some of specific
technologies involved in the preferred embodiment are expected to
change as techniques evolve through time. It should be understood
that the present invention is broad in concept and terms and is not
limited to the specific fabrication steps or final configurations
shown here. Other configurations could be generated for various
applications using known fabrication steps, and with new
fabrications techniques as they evolve.
As will be understood by those familiar with the art, the present
invention may be embodied in other specific forms methods of
manufacture without departing from the spirit or essential
characteristics thereof. Therefore, the scope of the present
invention is limited only by the scope of the claims appended
hereto.
* * * * *